This invention relates to volume and tone control circuits typically found in high fidelity sound reproduction systems and more particularly to a system for providing bias control signals to the volume and tone control circuits while substantially reducing noise produced by the control circuits.
Most, if not all, analog direct current controlled tone and volume control circuits use differentially connected transistor pairs as variable signal splitting circuits. For example, typical treble and bass tone control circuits are disclosed in U.S. Pat. No. 4,055,818, Michael J. Gay, which is assigned to Motorola, Inc. The amount of signal splitting between respective outputs of the transistor pairs is controlled by the direct current bias voltage supplied at the control terminals thereof.
A problem occurs with this arrangement by the generation of partition noise, i.e., the random variations in the division of the currents between the outputs of the two transistors of the transistor pair. From analysis, it has been determined that these noise sources have components which increase in amplitude as the bias current through the transistor pairs is increased. Since the bias current must be sufficient to carry the maximun input signal current supplied to the sound reproduction system, it is evident that the higher the input signal is, the higher the bias current must be and, hence, the higher the noise will be.
An audio sound system must normally accept input signals over a wide dynamic range. Under this condition the signal splitting volume and tone control circuits must be biased to carry the maximum signal levels and will therefore generate noise correspondingly.
Therefore, a need exists to provide circuitry and method for minimizing the noise produced in volume and tone control circuits of a high fidelity sound system which is generated by the magnitude of bias splitting currents produced therein.